When performing drilling or cutting operations downhole, it is desirable to be able to monitor and control the drilling or cutting process. However, in practice, this is difficult to achieve for several reasons. Firstly, it is difficult to know the exact position of a drill bit or cutting blade in the well and thus to determine exactly which part of the casing is being cut or drilled. Secondly, the drilling or cutting process cannot be visually inspected, and it is difficult to determine whether the machinery is operating properly based on known techniques. Furthermore, the specifications, composition or state of the component to be drilled in downhole may not always be known, or may prove to be different than expected, and may therefore not be as easy to drill into as expected. It would therefore be advantageous to be able to determine whether the correct weight on bit and drill bit rotary speed is applied and/or to monitor whether the drilling or cutting process proceeds as planned and whether unforeseen conditions occur.